1. Field of Invention
The present invention relates to a power supply input voltage detection circuit; particularly, it relates to such input voltage detection circuit which detects a signal related to the rising time of the input voltage to determine whether an input voltage is a high voltage or low voltage.
2. Description of Related Art
FIGS. 1 and 2 show schematic diagrams of prior art input voltage detection circuits for power supply control circuits 11 and 13, respectively. As shown in FIG. 1, the power supply converts an input voltage Vin to an output voltage Vout by a transformer 10. The transformer 10 includes a primary winding W1 and a secondary winding W2. A power switch Q1 is coupled to the primary winding W1, and it receives a switching signal from a switching terminal GAT to adjust the output voltage Vout. A resistor Rcs is coupled to the primary winding W1, and it generates a current sense signal according to current through the primary winding W1. The current sense signal is inputted to a current sense terminal CS of the control circuit 11 or 13. The control circuits 11 and 13 control the power supplies according to the current sense signal.
The prior art shown in FIG. 1 senses a current signal related to the input voltage Vin by coupling a resistor R1 to the input voltage Vin, and duplicates the current signal by a current mirror. A resistor R2 is connected to one branch of the current mirror in series to obtain a voltage across the resistor R2, which is a signal αVin related to the input voltage Vin. The signal αVin is used to determine whether the input voltage Vin is a high voltage or low voltage. The high and low voltages refer to voltages which a general electronic product will be in contact with during operation. As one example, the high voltage is around 265V AC and the low voltage is around 90V AC, but they may vary depending on regions.
The prior art shown in FIG. 2 senses the current signal related to the input voltage Vin by coupling a resistor R3 and a resistor R4 in series to the input voltage Vin, and obtaining the signal αVin related to the input voltage Vin by the divided voltage across the resistor R4, which is used for determining whether the input voltage Vin is a high voltage or a low voltage.
In both the aforementioned prior art control circuits 11 and 13, a specific node is required to Obtain a voltage or current signal related to the input voltage Vin. This means that an extra pin is required if the control circuits 11 and 13 are integrated into integrated circuits (ICs) respectively, and the manufacturing cost and space for the ICs are increased. Besides, both the aforementioned prior art circuits require additional resistors (R1 or R3 and R4) coupled to the input voltage Vin for obtaining the signal related the input voltage Vin, and this will increase power consumption and manufacturing cost.
In view of above, to overcome the drawbacks in the prior art, the present invention proposes a power supply input voltage detection circuit which determines whether the input voltage is a high or low voltage without requiring an extra pin of the integrated circuit.